1. Field of the Invention
This invention relates to an original reading imaging optical system and an image reading apparatus using the same, and particularly is suitable for reading a monochromatic image or a color image using the line sensor of an image scanner, a digital copier, a facsimile apparatus or the like using a compact imaging optical element in which various aberrations are well-balancedly corrected and which has high resolving power.
2. Related Background Art
As an image reading apparatus (image scanner) for reading image information on the surface of an original, a flat bed type image scanner is proposed, for example, in Japanese Patent Application Laid-Open No. 3-113961.
The flat bed type image scanner is such that an imaging lens and a line sensor are fixed and only a reflecting mirror is moved to thereby slit-expose and scan the surface of an original and read image information thereon.
In recent years, in order to achieve the simplification of the structure of the apparatus, it has often become the case that an integral carriage type scanning system in which a mirror, an imaging lens, a line sensor, etc. are made integral with one another to thereby scan the surface of an original is adopted.
FIG. 32 of the accompanying drawings is a schematic view of the essential portions of an image reading apparatus of the integral carriage type scanning system according to the prior art. In FIG. 32, a beam emitted from an illuminating light source 1 illuminates an original 8 directly placed on an original supporting table 2, and the reflected beam from the original 8 has its optical path bent in the interior of a carriage 6 through first, second and third reflecting mirrors 3a, 3b and 3c in the named order, and is imaged on the surface of a line sensor 5 by an imaging lens (imaging optical system) 4. The carriage 6 is moved in the direction of arrow A (sub scanning direction) indicated in FIG. 32 by a sub scanning motor 7 to thereby read the image information of the original 8. The line sensor 5 in FIG. 32 is of a construction in which a plurality of light receiving elements are arranged in one-dimensional direction (main scanning direction).
FIG. 33 of the accompanying drawings is an illustration of the basic construction of the image reading optical system of FIG. 32.
In FIG. 33, the reference numeral 4 designates an imaging optical system, the reference characters 5R, 5G and 5B denote line sensors for the respective colors R(red), G(green) and B(blue) of the line sensor 5, and the reference characters 8R, 8G and 8B designate reading ranges on the surface of the original corresponding to the line sensors 5R, 5G and 5B. In the image reading apparatus shown in FIG. 32, the surface of the stationary original is scanned by the carriage 6, but the carriage scanning is equivalent to the line sensor 5 and the imaging lens being stationary and the surface of the original 8 being moved, as shown in FIG. 33. By the surface of the original being scanned, the same location can be read in different colors at a certain time interval. When in the aforedescribed construction, the imaging lens 4 comprises an ordinary refracting system, on-axis chromatic aberration and chromatic aberration of magnification occur and therefore, relative to the reference line sensor 5G, defocus or positional deviation occurs to line images formed on the line sensors 5B and 5R. Accordingly, when images of the respective colors are superimposed and reproduced, the images become images in which color oozing and misregister are conspicuous. That is, when such performance as high aperture or high resolution is required, the requirement cannot be coped with.
On the other hand, recently, in a non-coaxial optical system as well, it has become apparent that the concept of a reference axis is introduced and a constituent surface is made into an asymmetrical aspherical surface, whereby an optical system in which aberrations have been sufficiently corrected can be constructed. The example, a designing method for it is shown in Japanese Patent Application Laid-Open No. 9-5650, and examples of the design thereof are shown in Japanese Patent Application Laid-Open Nos. 8-292371 and 8-292372.
Such a non-coaxial optical system is called an off-axial optical system (which is an optical system defined as an optical system containing a curved surface (an off-axial curved surface) in which, when considering a reference axis along a ray of light passing through the center of an image and the center of a pupil, a surface normal at the point of intersection of a constituent surface with the reference axis is not on the reference axis, and in this case, the reference axis is of a bent shape). This off-axial optical system is such that the constituent surface thereof generally is non-coaxial and eclipse does not occur even on the reflecting surface thereof and therefore, an optical system using a reflecting surface is easy to construct. It also has the features that the drawing around of an optical path can be effected relatively freely and that an integral type optical system is easy to make by the technique of integrally molding a constituent surface.
On the other hand, high resolution and a high speed are required of an original reading system such as a digital copier and therefore, such system has not yet come to be constituted by an integral type optical system. An imaging lens required in the reading system needs to be bright and high in resolution and therefore, to secure optical performance, it is difficult to make the angle of field great. If the angle of field is narrow, the optical path length will become long as the result.
On the other hand, when the reading of a color image is to be effected, as the resolution becomes higher, the imaging position difference for each color by chromatic aberration, and chromatic aberration such as color misregister in an image field adversely affect optical performance.